mkefidisk: Always specify a root= kernel parameter

mkefidisk: Always specify a root= kernel parameter

The current script only replaces an existing root= kernel parameter
which can result images created without a root= paremeter, even though
the script expects a target rootfs parameter.

Rather than replacing the root= parameter, delete the root= parameter if
it exists, then append an appropriate root= parameter.

Fixes [YOCTO 4035] mkefidisk.sh forgets to add root= parameter

(From OE-Core rev: e5dbec7e7d3bb29676280823b0337ad429c75120)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>